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to the significance of the world. It is one way in which those activities which make the real express themselves. It has its place or reason in the totality of things, and this whether we consider its origin or its position with regard to knowledge. In a word, while the characteristic of Locke’s theory is that he conceives sensation as in external relation both to reality, as mechanically produced by it, and to knowledge, as being merely one of the atomic elements which may enter into a compound, Leibniz regards reality as organic to sensation, and this in turn as organic to knowledge. We have here simply an illustration of the statement with which we set out; namely, that the treatment of sensation always reflects the fundamental philosophical category of the philosopher.
All reality exists in the form of monads; monads are simple substances whose nature is action; this action consists in representing, according to a certain law of succession, the universe. Various monads have various degrees of activity; that is, of the power of reflecting the world. So much of Leibniz’s general philosophical attitude it is necessary to recall, to understand what he means by “sensation.” The generic name which is applied to this mirroring activity of the monads is “perception,” which, as Leibniz often says, is to be carefully distinguished from apperception, which is the representation become conscious. Perception may be defined, therefore, as the inclusion of the many or multiform (the world of objects) in a unity (the simple substance). It was the great defect of previous philosophy that it “considered only spirits or self-conscious beings as souls,” and had consequently recognized only conscious perceptions. It had been obliged, therefore, to make an impassable gulf between mind and matter, and sensations were thus rendered inexplicable. But Leibniz finds his function as a philosopher in showing that these problems, which seem insoluble, arise when we insist upon erecting into actual separations or differences of kind what really are only stages of development or differences of degree. A sensation is not an effect which one substance impresses upon another because God pleased that it should, or because of an incomprehensible incident in the original constitution of things. It is a higher development of that representative power which belongs to every real being.
Certain monads reach a state of development, or manifestation of activity, which is characterized by the possession of distinct organs. Such monads may be called, in a pre-eminent sense, “souls,” and include all the higher animals as well as man. This possession of differentiated organs finds its analogue in the internal condition of the monad. What appears externally as an organ of sense appears ideally as a conscious representative state which we call “sensation.” “When,” Leibniz says, “the monad has its organs so developed that there is relief and differentiation in the impressions received, and consequently in the perceptions which represent them, we have feeling or sensation; that is, a perception accompanied by memory,” to which at other times he adds “attention.” Life, he says, “is a perceptive principle; the soul is sensitive life; mind is rational soul.” And again he says in substance that when the soul begins to have interests, and to regard one representation as of more value than others, it introduces relief into its perceptions, and those which stand out are called “sensations.”
This origin of sensations as higher developments of the representative activities of a monad conditions their relation to further processes of knowledge. The sensations are confused knowledge; they are ideas in their primitive and most undifferentiated form. They constitute, as Leibniz somewhere says, the vertigo of the conscious life. In every sentient organism multitudes of sensations are constantly thronging in and overpowering its distinct consciousness. The soul is so flooded with ideas of everything in the world which has any relation to its body that it has distinct ideas of nothing. Higher knowledge, then, does not consist in compounding these sensations; that would literally make confusion worse confounded. It consists in introducing distinctness into the previously confused sensations,—in finding out what they mean; that is, in finding out their bearings, what they point to, and how they are related. Knowledge is not an external process performed upon the sensations, it is the development of their internal content.
It follows, therefore, that sensation is organic to all forms of knowledge whatever. The monad, which is pure activity, that which culminates the scale of reality, has no confused ideas, and to it all knowledge is eternally rational, having no sensible traces about it. But every other monad, having its activity limited, has ideas which come to it at first in a confused way, and which its activity afterwards differentiates. Thus it is that Leibniz can agree so heartily with the motto of the Sensationalist school,—that there is nothing in the intellect which was not first in the sensory. But Leibniz uses this phrase as Aristotle would have done, having in mind the distinction between potentiality and actuality. In posse, sensation is all knowledge; but only in posse. And he, like Aristotle, interprets the relation between potentiality and actuality as one of a difference of activity. The potential is that which becomes real through a dynamic process. The actual is capacity plus action. Sensation, in short, is spiritual activity in an undeveloped and hence partial and limited condition. It is not, as Locke would have it, the real factor in all knowledge.
The marks of sensation which Locke lays down,—their passivity, their simplicity, their position as the real element in knowledge,—Leibniz either denies, therefore, or accepts in a sense different from that of Locke. Strictly speaking, sensation is an activity of the mind. There are no windows through which the soul receives impressions. Pure passivity of any kind is a myth, a scholastic fiction. Sensation is developed from the soul within; it is the activity of reality made manifest to itself. It is a higher kind of action than anything we find in minerals or in plants. If we look at sensation ideally, however, that is, according to the position which it holds in the system of knowledge, it is properly regarded as passive. It represents the limitation, the unrealized (that is, the non-active) side of spiritual life.
“Efficient causality” is a term which has its rightful and legitimate use in physical science. Simply from the scientific point of view we are correct in speaking of objects as affecting the body, and the body, through its nervous system, as affecting the soul and producing sensations. But philosophy does not merely use categories, it explains them. And Leibniz contends that to explain the category of causality in a mechanical sense, to understand by it physical influence actually transferred from one thing to another, is to make the idea inexplicable and irrational. The true meaning of causality is ideal. It signifies the relative positions which the objects concerned have in the harmonious system of reality. The body that is higher in the scale impresses the other; that is to say, it dominates it or gives its law. There is no energy or quality which passes physically from one to the other. But one monad, as higher in the stage of development than another, makes an ideal demand upon that one. It places before the other its own more real condition. The less-developed monad, since its whole activity consists in representing the universe of reality, answers to this demand by developing the corresponding quality in itself. The category of harmonious or co-operative action is thus substituted for that of external and mechanical influence. Physical causality when given a philosophic interpretation means organic development. The reality of a higher stage is the more active: the more active has a greater content in that it mirrors the universe more fully; it manifests accordingly more of the law of the universe, and hence has an ideal domination over that which is lower in the scale. It is actually (that is, in activity) what the other is potentially. But as the entire existence of the latter is in representing or setting forth the relations which make the world, its activity is aroused to a corresponding production. Hence the former is called “cause,” and the latter “effect.”
This introduces us to the relation of soul and body, or, more generally stated, to the relation of mind and matter. It is the theory of co-operation, of harmonious activity, which Leibniz substitutes for the theory which Descartes had formulated, according to which there are two opposed substances which can affect each other only through the medium of a deus ex machina. Locke, on the other hand, took the Cartesian principle for granted, and thus enveloped himself in all the difficulties which surround the question of “mind and matter.” Locke wavers between two positions, one of which is that there are two unknown substances,—the soul and the object in itself,—which, coming in contact, produce sensations; while the other takes the hypothetical attitude that there may be but one substance,—matter,—and that God, out of the plenitude of his omnipotence, has given matter a capacity which does not naturally belong to it,—that of producing